1. Field of the Invention
The present invention relates to a tool and a process for use in manufacturing or packaging of microelectronic elements, e.g., semiconductor wafers and chips, and more specifically to a bond metal injection tool, such as used for filling a mold with solder, and a process for forming bumps on a microelectronic element.
2. Description of the Related Art
“Flip-chip packaging” refers to a technique in which a microelectronic element such as semiconductor chip is placed face-down on a chip carrier with the contacts at the face of the chip bonded to contacts of the chip carrier. In a typical process, solder bumps are formed on the contacts of semiconductor chips while the chips are still attached together at dicing lanes in form of a wafer. Such process can be referred to as “wafer bumping”. Injection Molding Soldering (IMS) is a technique developed to address the cost vs. quality issues associated with current wafer bumping technologies. IMS applied to wafer bumping has been referred to as “C4NP” (Controlled Collapse Chip Connection New Process) by International Business Machines Corporation.
C4NP involves a bond metal injection tool 1 filling specially designed pits or “cavities” 14 at a major surface 12 of a mold 10 (FIG. 1) using a fill head 20 which has a chamber 22 containing molten solder 24 and a nozzle 26 for ejecting the solder. The mold 10 can be a removable “mold plate” superposed upon a hot plate 50, which is chosen for its thermal conductivity. The mold plate 10 typically is not wettable by solder and can be formed of a material such as borosilicate glass which is selected for its transparency in permitting optical inspection and for its thermal properties similar to silicon. The fill head has a nozzle 26 through which molten solder is injected into the cavities of the mold. For example, air pressure within the chamber can force molten solder outward through the nozzle. When the cavities are being filled, a hot plate 50 supporting the mold plate typically is maintained at a temperature below the melting temperature of the solder, so as to maintain the solder in the mold plate cavities below the melting temperature.
When the head 20 has finished filling a particular mold plate, it is moved onto a “parking space” 30 where it stays temporarily, with the molten solder present in the chamber and nozzle. The surface 32 of the parking space can be raised somewhat (e.g., 0.2 to 1.5 mm) above the major surface 12 of the mold plate. A gap 34 of less than a millimeter is typically disposed between an edge surface 36 of the parking space and an adjacent edge surface 16 of the mold plate when the parking space is ready for the head 20 to be moved from the parking space to the mold plate, or when the head is moved from the mold plate onto another parking space (not shown). The mold plate and the parking space can be separated from each other after the solder head is moved from the mold plate onto the parking space.
While the head rests on the parking space as seen in FIG. 1, the solder is constrained to the space between a surface 28 of the fill head and a confronting surface 32 of the parking space by a seal 23. Likewise, when the head is being used to fill the cavities as seen in FIG. 2, the solder is constrained between the fill head surface and the major surface 12 of the mold plate. One difficulty occurs when the head moves between parking space and the mold plate. The transition across the gap can cause solder to be drawn out from the nozzle 26 or shaken loose therefrom, leaving a mass 40 (FIG. 2) of solder at the gap between the parking space and mold plate or leaving a streak 42 of solder on the major surface 12 of the mold plate. Such masses and streaks, which are potential sources of defects, must be cleaned from the mold plate and the parking space. This impacts throughput and quality in the mold-filling operation.
Another problem is that solder oxidizes when it is molten and exposed to oxygen. Oxidized solder, if allowed to enter the cavities, can cause the bumps made of that solder to be mechanically fragile and ultimately, can increase electrical resistance in the solder bump connections between chip and chip carrier. Solder 40, 42, that exits the nozzle when the head moves onto the mold plate can become oxidized and cause these potential defects.